Natural populations of Drosophila mediopunctata harbor X chromosomes (called sex-ratio, or "SR") such that XSRY males produce no Y-bearing sperm. The resulting female-biased sex ratio can be driven to the point of population extinction. However, these populations also have Y chromosomes that are suppressors of SR expression, such that XSRY suppressor produce equal numbers of sons and daughters. This system is an excellent experimental model to study the population genetics of Y chromosomes and selfish genetic elements. The populations have a stable, non-neutral Y-polymorphism (Y supressor / Y non-suppressor) which is unexpected on theoretical grounds. We will do further field collections to test a proposed hypothesis of stabilization of this polymorphism. Recent characterization of three dynein genes on the Drosophila Y chromosome allows us to quantify levels of polymorphism and dates of common ancestry of Y chromosomes of D. mediopunctata. XSR and Y suppressor chromosomes are widespread, and it is not known whether they appeared once (and then spread) or whether they appeared several times. This question is fundamental to evaluate the evolutionary significance of selfish genes: repeated origins of selfish genes suggest that they are a frequent threat, whereas a single origin implies that once a selfish gene arises by mutation, it is able to invade all populations of a species. We will answer this question by phylogenetic studies using DNA sequences: if suppressor Y chromosomes from distant populations form a cluster (separated from non-suppressor Y), then they had a single origin. An analogous reasoning applies to the XSR chromosomes. DNA sequences will also be used to estimate the age of these selfish genes, using analytical methods based on the molecular clock.